Polarization diversity radiator for phased arrays

ABSTRACT

A radiator of the open end (square) waveguide type, having diagonal ridges in its corners. An independent coaxial feed through the closed end of the waveguide provides excitation to each ridge. Opposite ridge pairs are excited together on a 0*/180* basis by hybrid circuits. Each of the two hybrid inputs then controls one opposite ridge pair as a unit. The inputs of the two hybrid circuits are controlled in relative phase by a two-bit digital phase shifter and hybrid ring circuit. The result is a polarization diversity capability with pulse-to-pulse controllability. The ridges are tuned by capacitance and inductance gaps at the feed end and matched at the aperture by quarter wave impedance transformers in the ridges.

United States Patent POLARIZATION DIVERSITY RADIATOR FOR PHASED ARRAYS 7Claims, 1 Drawing Fig.

Int. "011 13/00 Field of Search 343/756,

[56] References Cited UNITED STATES PATENTS 2,942,261 6/1960 Jones etal. 343/756 3,458,862 7/1969 Franks 3431786 Primary Examiner-EliLieberman AtIomeys-C. Cornell Remsen, Jr., Walter J. Baum, Paul W.Hemminger, Percy P. Lantzy and Thomas E. Kristofferson ABSTRACT: Aradiator of the open end (square) waveguide type, having diagonal ridgesin its comers. An independent coaxial feed through the closed end of thewaveguide provides excitation to each ridge. Opposite ridge pairs areexcited together on a 0/ l 80 basis by hybrid circuits. Each of the twohybrid inputs then controls one opposite ridge pair as a unit. Theinputs of the two hybrid circuits are controlled in relative phase by atwo-bit digital phase shifter and hybrid ring circuit. The result is apolarization diversity capability with pulse-topulse controllability.The ridges are tuned by capacitance and inductance gaps at the feed endand matched at the aperture by quarter wave impedance transformers inthe ridges.

Aw HYdR/D BIA/6 //V MICROSTR/P Q-B/T PHASE SAW-7E7 PATENTEI] SEP 1 mm3,603,' 987 HYBRID BIA/6 //V 2- DIME/VS/O/VA7 8 TE/PL/NE 2-5/7 FmssSH/FTER. AND wee/0 RING /A/ MICE 03 TR IP INVENTOR. QOBERT A. W/TTE M-%'.4 AGENT POLARIZATION DIVERSITY RADIATOR FOR PHASED ARRAYS BACKGROUND OFTHE INVENTION 1. Field of the Invention The invention relates to radarantennas and more particularly to antenna radiators capable of rapidelectrically controlled polarization diversity.

2. Description of the Prior Art In the prior art, polarization diversityhas been accomplished for individual radiators or arrays of radiators ina number of mechanical and electrical ways. Among the mechanical systemsis the provision of rotatable grating pairs in the near field. Such asystem is describe in US Pat. No. 2,930,040. A less flexible system forconverting the polarization of a linear array form linear to circular isdescribed in US. Pat. No. 2,800,657.

Electrical control of polarization may be said to require the phasecontrol of radiation fields in each of two orthogonal physicaldimensions in a plane normal to the direction of radiation. inherently,some type of fast acting electrically controlled phase shifting deviceis required. In discussing prior art, U.S. Pat. No. 2,982,960 refers tothe use of ferrite materials as the inductive elements of irises (withelectrical control of the magnetization of the ferrites in a slottedwave guide radiator arrangement for that purpose.

Mechanical systems suffer the obvious disadvantage of slow reaction timeso that polarization control in a pulse-to-pulse basis is out of thequestion. An electrically controlled slotted waveguide system is likelynot to be adapted to inclusion in an array in which control of thepolarization on an individual radiator basis is required. Electricallycontrolled systems, moreover, are not readily adaptable where a largenumber of radiators are to be included in a linear or two dimensionalarray, since the combination of the radiator, feed and RF phase (andsometimes amplitude) control devices constitutes a complex and bulkyarrangement difficult to incorporate into an array requiringclose-spaced radiating elements. SUMMARY OF THE INVENTION Thisspecification describes a radiating element which has polarizationdiversity capability. An arbitrarily selected polarization may beselected on a pulse-to-pulse basis. That is to say, in a pulse radar thepolarization could be controlled at a time to be one of a choice ofvertical, horizontal right-hand circular or left-hand circular, and forthe next pulse period could be switched to another of thesepolarizations.

The basic electrical logic upon which the invention is based is thatthis selection of polarizations may be accomplished by generatingindependent orthogonal polarizations of equal amplitude and adjustablerelative phase. It will be appreciated as this invention is described,that any elliptical polarization can be formed by using appropriatephase shifting, however, for simplicity of description, the embodimentto be described in detail is one capable of horizontal, vertical and twocircular polarizations as aforementioned.

In order to maintain the common phase center necessary to maintain phasecoherence in the polarized radiated field, two orthogonal field vectorsare generated in the same radiator. This radiator is an open end squarewaveguide with four corner ridges, each independently excited from anend launching coaxial feed arrangement. Opposite corner ridges areexcited as pairs, so that the electric fields existing diagonallybetween ridges can add vectorially in the waveguide. The ridges of onediagonal pair may be thought of as being energized by energy of (0,) and(0,,+l80) feed phase relationships. The other diagonal pair will then beenergized by (74 and (0 '+l80). A two dimensional hybrid ring preferablyof stripline construction to conserve space, supplies these phasesrelationships from two inputs of 0,, and 74,. It is these latter twosignals that, by their relative phase relationship, determine thepolarization of the radiation. A two-bit digital phase shifter (i.e.,one capable of 2 states) and hybrid ring, preferably in microstrip,provides the 0,, and 0., signals from a single RF input of '1; energy. Acontrol input to this phase shifter then presets the 0 and 0,, phaserelationship. This phase shifter is preferably of the digital latchingtype, but

may be of a type responsive to a continuous discrete controlof-statesignal (analog device).

The waveguide ridges act to lower the cutoff frequency of a squarewaveguide of given size. Thus, energy of a given wavelength willpropagate down and be radiated from the open end of a ridged guide ofsmaller cross section than would be required without the ridges. Thisfeature of the invention is important in that the radiator, althoughuseable as an isolated radiator, was conceived as a radiating elementfor a closespaced array and is especially useful in that application.Other features of the invention will be evident from the preferredembodiment description following.

BRIEF DESCRIPTION OF THE DRAWING A single drawing FIGURE accompanyingthis specification, depicts in exploded isometric (partially cutaway),the elements of a typical embodiment of the radiator, phase-splittinghybrid ring, and phase shifter according to the present inven tion.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before undertaking the detaileddescription with references to the FIGURE, it is deemed desirable tobriefly examine the effect of scanning array design on radiator spacingand therefore on radiator size.

In a typical phased (frequency scanning) array, the radiator spacing (dfor avoidance of grating lobes is defined as follows:

where A wavelength at highest frequency in scan program 0 maximum angleof scan referred to array normal (broadside) In the limit as 0approaches d, becomes equal to or slightly less than one-halfwavelength, allowing a safety factor for finite beam-widths. For lessthan M2 spacing of radiator elements, the square waveguide is limited tothe same dimension on a side. As such, it would be below cutoff. Toovercome this limitation, the unique corner ridge loading scheme wasdeveloped to lower the cutoff frequency of the waveguide. With respectto the field between each opposite pair of ridges, the other diagonaldimension of the square waveguide acts as an enlarged transversedimension, making possible the propogation and radiation from the openend of the square waveguide of waves which would otherwise not propagatebecause of the aforementioned cutoff limitation.

Since there would be little or no allowable space between radiators, ofthe type herein described when assembled in an array, the end launchingscheme for exciting the radiators is provided by a coaxial feed for eachridge through the back (closed) end of the waveguide body 1 (SeeFIGURE).

In the description of the complete device with respect to the FIGURE,the radiator itself will first be described.

The radiator body I is actually a length of square (cross section)waveguide fabricated form well-known thin conductive material. The saidwaveguide body 1 is closed at the feed end by a conductive bulkheadthrough which the typical coaxial connectors 8 and 9 are shown toextend. The open end, or

I aperture, is framed by a flange 2, which might actually be the planesurface of an array, including a number of these radiators. The fourcorner ridges within the body 1 will be seen to be 3, 4, 5, and 6.lnherently, there are two impedance adjustment problems in such adevice. The first is the tuning of the load seen by each coaxial feedlooking into the ridge which it excites. The second is that of matchingthe radiator aperture to free space. The first of these matchingtasks'is accomplished by adjustment of the combination of the ridge toback plate spacing c and notch h length and diagonal depth e from theinside corner of the body 1, as shown The second impedance matching taskis handled by the notch of depth a and axial length approximately (M4)(in terms of wavelength within the guide measured from the aperturetoward the back plate) provided in each of the ridges.

The actual design parameters of the entire device were empiricallydetermined since this appeared to be the most economical technique. Itis, of course, subject to reverse engineering" analysis using rigorousmethods.

The empirical design was undertaken realizing that a distributedcapacitance exists between ridges and that the perimeter of thewaveguide cross section appears as a distributed inductance. Theseparameters are related to cutoff frequency and characteristic impedanceby the following equations: 1

Z Q and loop formed by dimensions I and h. The ridge depth s is given bys w/zb2fl d where d may be (1' or d depending upon the exact location ofthe transverse plane in which the said inside perimeter is beingexamined. Since the inductive parameter is related to this insideperimeter and therefore to t and s, the turning effect of thecapacitance at c is interrelated in a complicated way because the saidcapacitance depends not only on the clearance c, but also on t and e,since these affect the area of the end of the ridge facing the said backplate at c.

In order to clarify the invention and the typical dimensionalrelationships, Table I below has been included to list dimensions for atypical embodiment of the invention for use in the frequency range of3.0 to 4.0 GHz. All Table 1 dimensions and tolerances are in inches.

TABLE 1 Physical Parameter Dimension and Tolerance 10.003 2 0.110 10.003f 2.760 10.004 3 1.150 10.004 II 0.290 10.003

The device built as described, and accordingly to the Table Idimensions, was tested and was found to provide at least 30 db.isolation between the orthogonal radiator feed pairs betweenapproximately 3.0 and 3.8 GHz. Voltage standing wave ratiosgenerally didnot exceed 1.75 over that same frequency range. The device is thusrelatively broad-banded and, accordingly, well adapted to use in afrequency scanning array.

Continuing the description with reference to the FIGURE, the center pinof the coaxial input 8 is illustrated at 7, and is typical of the otherthree inputs relating to ridges 4, 5, and 6. These coaxial feeds aresymmetrically located and the pairs are typically spaced 1.340 10.003inches diagonally in the embodiment built according to Table 1. Theparticular coaxial connectors used were of a type known in the trade as3 millimeter coaxial connectors and are typically for use in coaxialtransmission circuits of 50 ohms characteristic impedance. Thus, theridge notch design previously discussed is directed toward presentationof 50 ohms impedance (real) at each of the four inputs to the radiator.

Concerning the two-dimensional hybrid ring comprising 14 and 115, it isto be understood that these elements are readily constructed by personsskilled in this art from knowledge of performance requirements.

It will be apparent from the FIGURE that the 10 and 12 hybrid outputsmate with 8 and its diagonally o opposite connector on the radiatorbody. Similarly, Ill and 13 mate with 9 and its diagonal companion.Hybrid inputs 16 and 17 mate with 18 and 19, the phase shifter outputs6,, and 0 The RF input to the system has already been referred to as 0and is applied at 21.. Terminal 22 is intended as the digital controlsignal into 20. The nature of the 2-bit phase shifter and hybrid ringpackage 20 is also well known in the art and can be constructed by aperson skilled in this art from performance requirements only.

[1, depicts the typical performance 20 in a representative embodiment asherein described, as follows:

An important feature of the radiator in this invention is the symmetryof the entire system. The two independent TE modes generated betweenopposing ridge pairs are effectively isolated from each other over afairly broad band of frequencies as previously indicated. Accurateconstruction of the two dimensional hybrid ring and the phase shifter isimportant in respect to isolation of these fields. That is, the outputsat 10 and 12 (6, and B d-) should be precisely 180 apart in phase andequal in power level. The same applies to 6,, and 0,, +180 fromterminals 13 and 11. The quality oflinear and circular polarizationradiation modes will, moreover, depend on the accuracy of the four phaserelationships, 6,, available at 18 (with respect to 0,, at 19), as wellas on the physical symmetry of the radiator itself.

Obviously, when a number of radiators of the type described are to beassembled into a practical array, the economical manufacture of theindividual radiators becomes a matter of great importance. The brazing,machining, and hand assembly methods of the model shop are thereforereplaced by production techniques. For example, the radiator body withintegral ridges is adapted to methods such as investment casting, oreven die casting. The ridges have been designed with an internal taperto provide draft for just such an operation. In a hand made model of theradiator, no such taper was necessary, but the tuning notch details andcertain other dimensions were slightly different in view of theinterdependence of the various parameters as aforementioned. Draft ortaper of the wall of body 1 may be accomplished externally (variablewall thickness) if required. The casting operation assumes that flangeor array face 2 is subsequently attached.

In one embodiment, the radiators were formed as an accurately moldedepoxy resin part with integral ridges, etc. A process for providing aconductive surface (necessary only inside) included processing steps ofconditioning the epoxy surface for electroless plating, applying theelectroless plating, copper electroplating to a predetermined thickness,and finally, the addition of a surface overcoating of electrolyticsilver or other electrically and environmentally suitable metal.

The draft provided as indicated in the FIGURE is necessary to this typeof process.

Numerous variations in the design are possible within the teachings ofthe invention described in a typical embodiment. The device may, ofcourse, be suitably designed for operation in some other frequency band.

The drawings and description are illustrative and representative, butare not intended to limit the scope of the claims.

What I claim is:

I. An antenna radiating element for rapid control of the polarization ofradiated energy, comprising:

a hollow conductive body comprising a length of square waveguide open atone end to form a radiating aperture, and closed at the opposite end;

four conductive ridges within said waveguide, one conductively fixed ineach comer of said waveguide, said ridges each lying in a plane passingthrough the center of the square cross section of said waveguide;

a separate feed connected to each of said ridges through said oppositeend of said conductive body; phase splitting means for energizingopposite pairs of said ridges in 180 phase relationship through saidridge feeds;

and means for controlling the relative phase of energy exciting each ofsaid pairs of ridges with respect to the other pair, thereby to controlthe polarization of energy propagated by said waveguide and radiatedfrom said open end.

2. The invention set forth in claim 1 in which said ridges extendlengthwise in their respective comers substantially from said open endto the closed end of said square waveguide within a predetermined firstair gap, and further, each of said ridges includes a second air gap inthe form of an undercut for a portion of its length measured from saidfirst air gap, said second air gap being between each ridge and thecorrespond- 4. The invention set forth in claim 1 further defined inthat I said ridges each include a matching cut, such as to increase thediagonal clearance within the waveguide between opposite ridges for adistance measured along the length of said ridges from said open end,said distance being substantially equal to an electrical quarterwavelength within said guide.

5. The invention set forth in claim 1 rnttnetaefi'ni intit'nT said phasesplitting means for energizing opposite pairs of said ridges in 180phase relationship includes a pair of hybrid rings, one for each of saidopposite pairs of ridges, each of said hybrid rings having an input and0 phase and 0+1 outputs.

6. The invention defined in 5, in which said means for I controlling therelative phase of energy exciting each of said pairs of ridges includesa phase shifter having one radio frequency input, two outputs, which areconnected to the inputs of said phase splitting means, and a controlinput, said phase shifter providing signals at its outputs different inphase by an amount variable between zero and 360 in response tocorresponding signals at said control input. p 7. The invention setforth in claim 6 in which said phase shifter is defined as a 2-bitdigitally controlled device, whereby the phase difference between saidphase shifter outputs is controlled to be 0, or 270 corresponding to thefour control conditions provided by a 2-bit digital control signal.

1. An antenna radiating element for rapid control of the polarization ofradiated energy, comprising: a hollow conductive body comprising alength of square waveguide open at one end to form a radiating aperture,and closed at the opposite end; four conductive ridges within saidwaveguide, one conductively fixed in each corner of said waveguide, saidridges each lying in a plane passing through the center of the squarecross section of said waveguide; a separate feed connected to each ofsaid ridges through said opposite end of said conductive body; phasesplitting means for energizing opposite pairs of said ridges in 180*phase relationship through said ridge feeds; and means for controllingthe relative phase of energy exciting each of said pairs of ridges withrespect to the other pair, thereby to control the polarization of energypropagated by said waveguide and radiated from said open end.
 2. Theinvention set forth in claim 1 in which said ridges extend lengthwise intheir respective corners substantially from said open end to the closedend of said square waveguide within a predetermined first air gap, andfurther, each of said ridges includes a second air gap in the form of anundercut for a portion of its length measured from said first air gap,said second air gap being between each ridge and the correspondingcorner of said waveguide.
 3. The invention set forth in claim 2 in whichthe relative sizes of said first and second air gaps are apportioned sothat their effects on capacitance and inductance, respectively, aseffective at the points of said feeds, tend to be compensatory, therebyto effect tuning of said feeds.
 4. The invention set forth in claim 1further defined in That said ridges each include a matching cut, such asto increase the diagonal clearance within the waveguide between oppositeridges for a distance measured along the length of said ridges from saidopen end, said distance being substantially equal to an electricalquarter wavelength within said guide.
 5. The invention set forth inclaim 1 further defined in that said phase splitting means forenergizing opposite pairs of said ridges in 180* phase relationshipincludes a pair of hybrid rings, one for each of said opposite pairs ofridges, each of said hybrid rings having an input and theta phase andtheta +180* outputs.
 6. The invention defined in claim 5, in which saidmeans for controlling the relative phase of energy exciting each of saidpairs of ridges includes a phase shifter having one radio frequencyinput, two outputs, which are connected to the inputs of said phasesplitting means, and a control input, said phase shifter providingsignals at its outputs different in phase by an amount variable betweenzero and 360* in response to corresponding signals at said controlinput.
 7. The invention set forth in claim 6 in which said phase shifteris defined as a 2-bit digitally controlled device, whereby the phasedifference between said phase shifter outputs is controlled to be 0*,90*, 180*, or 270* corresponding to the four control conditions providedby a 2-bit digital control signal.